The present invention relates to a plate bending machine which bends a plate clamped between a lower die and a pressure die by a blade of a bending die mounted on a bend beam which is controlled in the vertical and horizontal directions, and particularly relates to the structure of such a bending section.
As shown in FIG. 4 illustrating a side view of a plate bending machine, for example, there is known a plate bending process which includes the steps of clamping a plate 1 between a lower die 3 and an upper pressure die 2 in cooperation with each other and providing a bend beam 110a on which a bending die 107 is mounted at the tip end thereof, the bending die 107 including an upward extending blade 132 and a downward extending blade 131, whereby the plate 1 can be upward-bent by upwardly pressing the plate through the blade 132 at the approaching position in the underside of the plate and can be reverse-bent by downwardly pressing the plate through the blade 131 at the approaching position in the upper portion of the plate.
In such an arrangement, the bend beam 110a includes a horizontal bend arm 111a connected at one end to a shaft A (or eccentric shaft 113a) and a vertical bend arm 112a having its lower end connected to another shaft D (or eccentric shaft 114a), the upper ends of the horizontal and vertical bend arms being coupled together by a pivot pin 115a. 
Therefore, the shafts A and D can cooperate with each other to control the bending trace in the blades.
FIG. 5 A is a front view of a main part of such a plate bending machine while FIG. 5 B is a top plan view of the same.
In these figures, the lower die portion is omitted for simplicity.
These figures schematically represent a linkage and crank mechanism for the sake of clarity.
The rotation of the shaft A (or eccentric shaft 113a) and shaft D (eccentric shaft 114a) is controlled by a drive means 21 such as a servo motor through a speed reducer 23 or the like.
Reference numerals 22 and 28 denote bearing mechanisms which are supported by the corresponding frames (not shown).
Since the structure shown in FIG. 4 can use the blade 132 for upward-bending in the upward direction and the blade 131 for reverse-bending in the downward direction, it is advantageous in that the blade to be used can be selected merely by rotating the eccentric shaft D (eccentric shaft 114a) without exchanging the bending dies which have different blades.
Since the forward end of the plate can more deeply be inserted into such a bending structure as shown in FIG. 4 without interference with the bending die, the width of the plate to be bent can be increased. However, the interference between the lower blade 131 and the lower die 3 must be avoided when the upward-bending trace is to be controlled by the upper blade 132.
Similarly, the interference between the upper blade 132 and the pressure die 2 must be avoided when the reverse-bending is to be carried out by the use of the lower blade 131.
It is also necessary that the eccentricity in the shaft D is equal to the sum of the movement of the bending die required to switch between the upward and reverse bending modes and the movement of the bending die required to perform the bending process. As a result, the bending machine requires a relative large power with increase of its size.
Such a structure as shown in FIG. 6 is also known.
This structure includes a bend beam 110b having its vertical cross-section of substantially inverted C-shaped configuration, and upper and lower bending dies 108 and 109 each of which includes a blade disposed to face the center.
The bend beam 110b including the bending dies mounted thereon is slidable along so-called LM guide blocks 124 and LM (Linear Motion) guide rails 125A in the vertical direction.
When the bending process is to be switched between the upward and reverse directions, the bending process can be carried out after the bending die 108 or 109 has been moved to a position near the plate through the LM guide mechanism without exchanging the bending die as in the arrangement of FIG. 4.
This structure of FIG. 6 is advantageous in that since the blades face the center, the rigidity can easily be ensured against the horizontal pressure from the horizontal bend arm 111b cooperating with the vertical bend arm 112b in its vertical movement when a plate is to be subjected to pushing-bending or ironing-bending.
Since the upward-bending die 109 is separated from the reverse-bending die 108, furthermore, it is relatively easy to avoid the interference of the bending die that is not used for bending.
Since the tip end of the plate interferes with the inner C-shaped wall of the bend beam 110b, however, the width of the plate to be bent is restricted.
Furthermore, the use of the LM guide mechanism causes shaking on bending. Additionally, the production cycle would be elongated since time required to perform the mode-switching step was relatively long in comparison with the structure of FIG. 4.